For what reason are you distinguishing them? I'd consider honor an R-stem for pretty much all intents and purposes, even though it started out S-stem, because its declension has been entirely regularized.
– DraconisJan 12 '17 at 4:03

@Draconis I am a beginner in Latin and don't want to lose sight of some distinction that may later become important. For example, I found that it was sometimes difficult to make sense of Greek noun declensions without knowing their earlier forms (e.g. the accent on the antepenult of πόλεως, from πόλῐος). I suppose you are telling me that the r/s will not have that sort of importance?
– CatomicJan 12 '17 at 4:19

1

Welcome to the site! I agree that the distinction will not be important in Latin. Elaborating on that would make a good answer. I would still like to know the answer to the question, not out of practical need but curiosity.
– Joonas Ilmavirta♦Jan 12 '17 at 6:49

2

Yeah, I think it was smart to do Greek first and then Latin. I did it the other way around and going from a deeply regular language to a deeply irregular one really did a number on me. There are a VERY few words in Latin whose original forms it's helpful to keep in mind, but if I count them off the top of my head I still have fingers left over.
– Joel DerfnerJan 12 '17 at 14:34

Duo was used commonly enough to escape this change, just like it preserved the dual endings which otherwise died out.

OS and OM at the end of a word became US and UM

Compare tempus (earlier *tempos) with temporis (earlier *temposis)

Or compare the Latin second-declension endings with the Greek ones

And many others...

As you can see, following these backward is not a trivial process. (Though it is very very interesting.)

Luckily...

This is almost never necessary.

Latin regularized forms much more than Greek did. In Attic Greek especially, inflections often don't make sense unless you know the underlying forms. But in Latin, these irregularities were smoothed out over time: lār and honor became normal R-stem nouns; flōs and tempus are normal R-stems except for the nominative singular.

So if you know the nominative singular, which is sometimes unusual, and the genitive singular, which never is (because the ending starts with a vowel), you know everything you need to decline the noun properly.

The general rule is that in Latin etymological s becomes r between two vowels. Thus nom. honos, but gen. honoris (but also nom. honor by analogy to the other cases). Words like honestus confirm that this is indeed an s-stem.